Electroplating solution containing organic acid complexing agent

ABSTRACT

A solution for use in connection with the deposition of one or more metals on electroplatable substrates. This solution includes water; a metal ion; and a complexing agent. The complexing agent is advantageously an organic compound having between 4 and 18 carbon atoms which includes at least two hydroxyl groups and a five or six membered ring that contains at least one oxygen atom. The compound is present in an amount sufficient to complex the metal in the solution and inhibit oxidation of the metal. If necessary, a suitable pH adjusting agent can be included in the solution to maintain the pH of the solution in the range of between 2 and 10 and preferably to a pH of about 3.5 to 5.5. At the preferred pH range, the solution is particularly useful for electroplating composite articles that have electroplatable portions and non-electroplatable portions without deleteriously affecting the non-electroplatable portions.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of U.S. provisional application No. 60/357,330 filed Feb. 15, 2002.

BACKGROUND OF THE INVENTION

[0002] The present invention relates to the deposition of metals and more specifically to the deposition of tin or tin-lead alloys on objects or articles composed of an electroplatable substrate, such as metal, or a composite article having electroplatable and non-electroplatable portions. The present invention also describes a method for inhibiting the fusing of a plurality of such composite articles during electrodeposition. This is relevant to the electroplating of small electrical components that have large surface areas per unit mass and are susceptible to fusing. Of particular interest herein are electrical components such as surface mounted capacitors and resistors that have metal portions as well as ceramic, glass, or plastic portions.

[0003] The size of electronic components has been dramatically reduced in recent years. This reduction in size has made these components significantly more difficult to electroplate. Additionally, many surface mount technology (SMT) components have sensitive ceramic portions which can be damaged by highly acidic or highly alkaline solutions. To avoid this problem, neutral or near neutral pH electroplating solutions are desirable.

[0004] Neutral or near neutral pH tin and tin/lead alloy electrolytes that are specifically formulated to be compatible with sensitive ceramic SMTs are described in U.S. Pat. Nos. 4,163,700, 4,329,207, 4,640,746, 4,673,470 and 4,681,670. The formulations described in these patents include complexing agents of components such as citrates, gluconates, or pyrophosphates to complex the tin and/or lead and render them soluble in the solutions at the elevated pHs required.

[0005] The use of prior art solutions has a persistent problem of component coupling or agglomeration during electrodeposition. It is quite common when tin or tin alloy plating small components with flat surfaces that the components tend to cluster together during plating. It is not uncommon when barrel plating SMT components that up to 10% of the load may be coupled (i.e., stuck together). Under some conditions, the entire load fuses together in large lumps. The extent of this problem depends on the plating solution composition as well as plating method and geometry of the components. This problem is particularly pronounced in tin-lead alloy electroplating

[0006] The neutral or near neutral pH tin and tin/lead alloy electrolytes that are specifically formulated to be compatible with sensitive ceramic SMTs have some utility, but they do not address the issue of part fusing. This is a particularly significant problem when the parts are of relatively small size. The present invention now provides a solution and process that overcomes this problem.

SUMMARY OF THE INVENTION

[0007] The present invention relates to a solution for use in connection with the deposition of one or more metals on electroplatable substrates. This solution comprises water; a metal ion in an amount sufficient to provide a metal deposit on a platable substrate; and a complexing agent. The complexing agent is advantageously an organic compound having between 4 and 18 carbon atoms which includes at least two hydroxyl groups and a five or six membered ring that contains at least one oxygen atom. The agent is present in an amount sufficient to complex the metal and render it soluble in the solution. In addition, the agent inhibits oxidation of the metal ion in the solution. When the metal ion has the ability to exist in the solution in at least two different valence states, the complex agent prevents oxidation of the metal from a lower valence state to a higher valence state. If necessary, a suitable pH adjusting agent can be included in the solution to maintain the pH of the solution in the range of between 2 and 10. At the most preferred pH range, the solution is particularly useful for electroplating composite articles that have electroplatable portions and non-electroplatable portions without deleteriously affecting the non-electroplatable portions.

[0008] The complexing agent preferably has one of the following the structures:

[0009] wherein each R is the same or different and is hydrogen or a lower alkyl group of 1 to 3 carbon atoms, T is R, OR, or O═P(OR)₂—, Z is O═ or RO—, n is 2-4 and Z can be the same or different in each occurrence in the structure, and m is 1-3, or the complexing agent is a soluble salt of such structure. The most preferred compounds include ascorbic acid, isoascorbic acid (also called erythorbic acid), dehydroascorbic acid, glucoascorbic acid, galacturonic acid, glucoronic acid, and glucose-6-phosphate, or a salt thereof. Typical salts include alkali or alkaline earth metals. These agents are generally present in an amount of about 25 to 200 g/l.

[0010] The invention also relates to a method for electroplating a metal deposit on composite articles that includes electroplatable and non-electroplatable portions. This method comprises contacting a plurality of such articles with one of the solutions described herein and passing a current though the solution to provide metal electrodeposits on the electroplatable portions of the articles without deleteriously affecting the non-electroplatable portions of the articles. Preferred metal electrodeposits are tin metal or tin-lead alloys and the preferred articles are electronic components.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] It has now been discovered that the fusing of the composite article electronic components can be largely eliminated by providing an electrolyte which includes one or more of the complexing agents disclosed herein. In particular, ascorbic acid and related compounds are the most preferred for use as such complexing agents.

[0012] The complexing agents are preferably utilized in solutions for electroplating tin or tin-lead deposits although they can also be used in solutions for electroplating other metals, particularly those metals that have multiple valence states. These complexing agents help maintain the metals in the solution at one of their lower valence states, thus facilitating the electroplating step and avoiding oxidation of the metals which can affect proper operation of the solution. Stannic tin is also complexed in these systems.

[0013] Any of the complexing agents of the formulae given above can be used in this invention. Advantageous complexing agents are organic acids, with preferred agents including ascorbic acid, isoascorbic acid, dehydroascorbic acid, glucoascorbic acid, galacturonic acid, and glucoronic acid. Salts of these acids can also be used, with the preferred salts being the alkali or alkaline metal salts. Ketogluconates can be used because these compounds convert in the bath to ascorbic acid. Heptagluconates are also suitable since they convert in the solution to similar acidic species. Any of these agents can be used at a typical amount of about 25 to 200 g/l. The most preferred complexing agent is ascorbic acid or an ascorbate salt because these compounds are relatively low cost and are readily available.

[0014] Ascorbic acid is included in the solution as simple ascorbic acid, an ascorbate salt such as sodium or potassium ascorbate, and/or as an ascorbic acid-metal complex, tin ascorbate for instance. The latter is preferred when it is desired to utilize other acidic components, such as organic acids or organic acid salts, to maintain the desired solution pH. The amount of ascorbic acid present should at a minimum be sufficient to render the metals present in the solution soluble at the given pH of the solution. As such the amount of ascorbic acid required is proportional to the metal concentration. At a tin concentration of 15 g/l, the preferred ascorbic acid concentration is about 45 to 200 g/l.

[0015] Any electroplatable substrates can be plated using the solutions of the present invention. Generally, these substrates are made of a metal such as copper, nickel, steel or stainless steel. In today's commercial products, many parts that require electroplating are being made in smaller and smaller sizes. In particular, electronic components are a typical example of such parts. Furthermore, these parts are composite articles that have electroplatable and non-electroplatable portions. While the metal portions are metals or metallic, the non-electroplatable parts are typically ceramic, glass or plastic. The present solutions are particularly useful for electroplating such composite articles.

[0016] The electroplating solutions can have any pH between 2-10, but preferably is in the range of about 3 and 7.5, and more preferably is about 4 to 5.5 so that the solution is compatible with the electronic components that are to be plated. When the components have metallic and inorganic portions, the preferred pH range enables metal to be deposited on the metallic portions without adversely affecting the inorganic portions. Generally, very high or very low pH solutions will damage the ceramic portions of the composite articles to be plated.

[0017] These solutions preferably do not contain appreciable amounts of free acid or free base, although essentially any acid or base can be used for pH adjustment. Generally, since the solution is acidic, a base or basic component is utilized to convert free acid to its corresponding salt. Preferred bases for this purpose include sodium or potassium hydroxide as well as many others.

[0018] The solution is formulated to be compatible with the substrates to be plated, and preferably to have no adverse effect on the substrates. When composite articles that have electroplatable and non-electroplatable portions are to be plated, the solution should be formulated to not attack or crack the non-electroplatable portions of the substrates. A simple test can be used to determine substrate/solution compatibility. The articles to be plated can simply be immersed in the proposed solution for a period of time that is equal to or longer than that which is to be used for the plating process. The temperature of the solution can be that which approximates the temperature of the solution during the plating process, or an elevated temperature can be used for an accelerated test. The parts are immersed in the solution for a desired time and then are recovered and weight to determine weight loss that occurs due to attack of the articles by the solution during immersion.

[0019] For example, composite articles used for capacitor manufacture now are being made with a low-fired ceramic. These ceramics contain a larger proportion of glass than conventional ceramics, and are more prone to attack during the plating process. A simple comparison test was made to determine the compatibility of various commercially available solutions and with a solution according to the present invention. The capacitors were placed into beakers containing equal amounts of these solutions, and weight loss of the parts after 5 hours immersion was measured. The results are shown in the following table: Weight loss after 5 hours Solution immersion in the solution (%) Competitor A (gluconate based 1.0% bath at pH of 3.5) Competitor A (gluconate based 0.5% bath at pH of 4) Competitor B (citrate based bath 5.0% with pH of 4.2) Present Invention (ascorbic acid 0.0% bath with pH of 5)

[0020] This table shows that the present invention has essentially no effect on the capacitors and is a substantial improvement for the plating of such components compared to conventional baths.

[0021] A particularly useful device for electroplating such electrical components is disclosed in U.S. Pat. No. 6,193,858, and need not be described further herein. To the extent necessary, the entire content of that patent is expressly disclosed herein by reference thereto.

[0022] Improvements to the previously patented system have been disclosed in Published International Application WO02/053809, the entire content of which is expressly incorporated herein by reference thereto. The immersion of the plating chamber into the electrolyte, as disclosed in this application, represents a significant improvement in that external soluble electrodes can now be used.

[0023] It has been found that electrolytes which contain the complexing agents of the present invention are capable of electrodepositing tin or tin-lead alloys while minimizing the fusing or coupling of the electroplated parts, as well as without deleteriously affecting the non-electroplatable portions of the articles. In this regard, these electrolytes are superior to those of the prior art, and in particular to baths that are citrate based. The complexing agent serves to maintain the tin and/or lead in solution at the pH of the electrolyte. Certain complexing agents, in particular ascorbic acid, also serves as a stabilizer for preventing the oxidation of stannous tin to stannic tin.

[0024] L-ascorbic acid (AA) readily converts to L-dehydroascorbic acid (DAA). In addition, DAA can easily return to AA by the conversion of two ketone groups to hydroxyl groups on adjacent carbons with the single bond connecting those atoms being converted to a double bond. The ease in which AA converts to DAA renders AA a strong reducing agent. In the plating solutions of the present invention, AA assists in complexing the tin ions both in their divalent and tetravalent states. This prevents or at least minimizes the formation of tin oxides that would precipitate to form sludge which deleteriously affects the performance of the solution.

[0025] A preferred solution according to the present invention comprises water, a divalent tin salt, and ascorbic acid as a complexing agent, and optionally contains a divalent lead salt, a salt to increase electrical conductivity, a surface active agent, or an agent to promote anode dissolution.

[0026] The stannous salts which may be used in this invention include stannous sulfate, stannous chloride, stannous oxide, stannous methane sulfonic acid, stannous ascorbate or any other suitable source of stannous tin. The stannous tin concentration in the solution maybe from 5 to 100 g/l and most preferably from 10 to 50 g/l. As noted above, the complexing agents of the invention also complex stannic salts, so that it is possible to add stannic salts to the solution instead of or along with stannous salts without concern.

[0027] The lead salts that may be optionally included to provide tin-lead deposits include any solution soluble divalent lead salt including, for example, lead methanesulfonate, lead acetate or lead ascorbate.

[0028] The conductivity of the solution maybe increased if necessary by the additional of a salt. If a pure tin solution is desired, a simple salt such as potassium sulfate may be used. If a tin-lead alloy is desired, potassium methanesulfonate or potassium acetate would be appropriate. Metal sulfide salts can also be used if desired. Any of these salts may be used to promote anode dissolution and assist in electrodeposition.

[0029] Surfactants which are typically utilized in tin or tin alloy electrolytes may be included in the solution to improve deposit crystalline structure and improve deposit quality at high current densities. Preferred surfactants include solution soluble alkylene oxide condensation compounds, solution soluble quaternary ammonium-fatty acid compounds, solution soluble amine oxide compounds, solution soluble tertiary amine compounds or mixtures thereof. One preferred surfactant is an alkylene oxide condensation compound and is present in an amount of about 0.01 to 20 g/l. Other conventional surfactants can be used as there is no criticality to this component with regard to deposit appearance, although some additives may perform better than others with regard to coupling of the articles to be plated. One of ordinary skill in the art can perform routine testing to determine the most appropriate surfactants for any particular plating solution.

[0030] When bright deposits are desired, an aromatic aldehyde can be added in an amount sufficient to act as a brightener. Other conventional brighteners can instead be used if desired.

[0031] The substrates to be electroplated are preferably those composite articles that have conductive and non-conductive portions. While the metal portions are metals or metallic, the non-conductive parts are typically ceramic, glass or plastic. The present solutions are particularly useful for electroplating such composite articles without deleteriously affecting the non-metallic portions of the articles and without causing fusing of such parts.

[0032] The pH of the electrolyte is preferably retained in the range of about 4 to 5.5 when plating on composite substrate electronic components is desired. The pH can be raised by the addition of caustic, for example potassium hydroxide, ammonium hydroxide, sodium hydroxide or the like, or can be lowered with an acid such as sulfuric or methanesulfonic. An alkane or alkanol sulfonic acid, such as methanesulfonic acid, is preferred for tin-lead alloy solutions, since sulfuric acid can generate lead sulfate which is insoluble in the solution and which would tend to precipitate. As noted above, a pH of about 4 to 5.5 results in the strongest inhibition of agglomeration of such metals. Furthermore, the amount of ascorbic acid should not be in great excess to that needed to complex the tin in order to inhibit and minimize agglomeration.

[0033] Typical antioxidants used in tin and tin-lead solutions may be included in the solution of the present invention (e.g., catechol or hydroquinone as disclosed in U.S. Pat. No. 4,871,429), however ascorbic acid has been found to be effective in preventing the oxidation of stannous tin to stannic tin in neutral or near neutral pH plating solutions. As such, ascorbic acid serves the dual function of acting both as a complexing agent and as an antioxidant in the present solutions.

[0034] It has also been discovered that the plating of the non-conductive portions of composite articles and the fusing of the composite articles can be minimized or largely eliminated by fomulating the plating solutions of the present invention to have a low throwing power. These solutions are specifically formulated to not deposit metal at low current densities. This is contrary to conventional practice where electrolytes are formulated to deposit metal at as broad a current density range as possible. In fact most conventional tin electroplating solutions go to great lengths to extend the current density range of electrodeposition by adding various additives. It has now been found that fusing of parts can be minimized by limiting the current density range of electrodeposition to higher current densities. It is believed that fusing occurs due to metal deposition in the electrolyte film between two parts in close contact or between the parts and the current feeder. Because the deposition occurs in a thin film between two conductive surfaces, it necessarily occurs at low current densities. By formulating the electrolyte to not plate at low current densities, fusing can be minimized.

[0035] It has been found that part fusing is closely dependent on the plating bath composition and selection of the proper grain refiner or surfactant is critical to minimizing fusing. In this regard, simple electrolytes which contain only the metal salt and a complexing agent have been found to electroplate surface mount technology (SMT) components without fusing. The resulting tin deposit is a dark gray matte and is not acceptable for commercial use. When typical surfactants or grain refiners are added to the electrolyte to improve the quality of the deposit, in almost all instances very strong fusing is observed. It appears that the cathode surface polarization resulting from surfactants and grain refiners strongly influences part fusing. Moreover, it has been found that electrolytes containing additives which impart limited coverage at low current densities are less prone to fusing than additives which impart high coverage at low current densities.

[0036] It is a widely held belief that solutions formulated for plating of discrete components in barrels or other suitable equipment must have high throwing power so that current will penetrate the load and deposit metal within the bulk of the load. It is also clear that plating speeds are so negligible at low current densities that no substantial amount of metal is deposited under these conditions, rather the majority of the metal is deposited at high current densities near the plating barrel circumference. Therefore, it has now been found that it is unnecessary to provide a solution with high throwing power for plating discrete components in a barrel or other suitable apparatus as long as the parts themselves have no low current density areas such as hollows or blind holes.

[0037] Furthermore, a plating solution that does not deposit metals at low current densities will minimize the deposition of metal on the non-conductive portions of composite articles. The phenomenon of metal deposits extending from the conductive termination of the article onto the non-conductive portions is commonly referred to as creep or bridging. The extent of this phenomenon is primarily dependent on the composition of the non-conductive material. For example, ceramic materials that have some electrical conductivity are more prone to metal creep than ceramic material that are perfect insulators. It is believed that creep is caused by electrical currents leaking from the conductive portions of the article into the “non-conductive” composite portions during electrodeposition. By restricting metal deposition to high current density conditions, the deposition of metal onto the non-conductive portions can be minimized or eliminated.

[0038] The plating solutions of the present invention can be and preferably are formulated to have the following attributes and advantages:

[0039] 1. they will deposit a white matte to a semi-bright deposit.

[0040] 2. they will not damage the component to be coated.

[0041] 3. they will not deposit metal at low current densities.

[0042] When the parts to be plated are composite articles containing ceramic or leaded glass portions, acid or alkaline solutions will damage the ceramic or glass portions during electroplating. Components such as SMT resistors, inductors and capacitors are of this type. The pH of the electroplating solutions for SMT components must be between about 2.5 and 9 in order to minimize damage to the ceramic or glass portions of the article. To achieve this pH, the tin must be in a complexed form. Prior art complexing agents typically include citrates, gluconates, and pyrophosphates. In order to plate a semi-bright deposit, however, one or more organic additives are typically used. Most common additives greatly increase the low current density coverage of the solution resulting in fusing of the parts to be plated and overplating of the non-conductive portions of the parts.

[0043] The low current density coverage of an electrolyte may be reduced by operating the electrolyte at high metal concentrations, operating at elevated temperatures, selecting additives which do not increase low current density coverage (LCDC) or which reduce the LCDC and/or any combinations of the same. For example, a high metal ion content of at least about 25 g/l is preferred when tin is the metal to be electroplated. As it has been found that high temperatures generally increase parts fusing, elevated bath temperatures are the least desirable method of decreasing the LCDC.

[0044] Selection of the organic additives in the bath is particularly important in maintaining the throwing power at a low level. The most desirable additives can be determined by routine testing on the specific plating solution of interest. These additives include conventional surfactants and grain refiners, such as condensation compounds of organic compounds such as single or multiple aromatic rings as well as other organic condensation or reaction products that have dye-like properties but which are not surfactants. These compounds are generally known in the art and are simply tested to assure that they do not impart a high throwing power to the plating solution.

[0045] Other additives may be used in combination with surfactants and grain refiners to reduce the throwing power of the electrolyte. Ammonium chloride, ascorbic acid, and M-nitro phenol have been found to reduce throwing power when used in conjunction with various surfactants and grain refiners. Clearly, many other additives would function in this way and the use of these other additives is a subject of the present invention. One of ordinary skill in the art can conduct routine testing to determine the best combination of additives to use or not to use for any particular electroplating solution.

[0046] When plating composite articles, the present solutions can be used in the equipment disclosed in U.S. Pat. No. 6,193,858 and published International Application WO02/053809. The plating solutions of the present invention can also be used in the rotary plating apparatus described in U.S. Pat. Nos. 5,487,824 and 5,565,079 with improved results, since the deposition of tin on the current feeder ring is substantially reduced, resulting in significant reductions in maintenance required to replace and strip the current feeder.

[0047] Therefore, the use of electrolytes with reduced LCDC in rotary plating apparatus is also a subject of the present invention. The use of the current invention is also advantageous when using plating barrels since less metal will be deposited on the danglers, and metal creep and parts fusing are reduced. Therefore, the use of the LCDC electrolyte in barrel plating is also a subject of the current invention.

[0048] Although the present invention is particularly advantageous when plating composite components without media, the use of the present invention to plate discrete articles mixed with media has the significant advantages of reducing plating on the current feeder and reducing or eliminating metal deposition on the non-conductive portions of the composite articles.

[0049] A useful method for testing an electrolyte for LCDC is to use a standard 265 ml hull cell test. The standard procedure for running a hull cell is used. Typical conditions would be 1 A for 5 minutes, 0.5 A for 5 minutes or 0.25 A for 5 minutes, each using paddle agitation. Hull cell panels prepared at 1 A have LCDC if the back of the hull cell panel is largely unplated except for portions extending less than 1 cm from the panel edge. Additionally, electrolytes which are most preferred will have an unplated portion on the front side low current density edge. This unplated portion may be from ⅛″ to {fraction (3/4)}″ of an inch wide. Electrolytes which exhibit this type of hull cell panel results normally are much less prone to fusing than electrolyte which produce significant plating on the back side of the hull cell panel. The limiting current density at which metal will not be deposited may be measured by preparing a hull cell panel at 0.25 A and determining the current density at the edge of the metal deposit using the appropriate Hull cell panel scale.

[0050] Additionally, when electrolytes with LCDC are used in the SBE apparatus to late SMTs without media, it is generally found that the current feeder is largely unplated by tin at the end of the plating cycle and that the parts to do not fuse to the current feeder. In contrast, when commercially available neutral tin plating electrolyte is used to plate SMTs in the SBE without media the parts seize within three minutes of the start of plating and the current feeder is found to be completely coated with tin. Therefore, the use of a tin or tin alloy electrolyte with LCDC is necessary for successful plating of SMTs without media in the SBE apparatus.

EXAMPLES Example 1

[0051] A pure tin electrodeposit is obtained from the following solution and under the following electroplating conditions. Ascorbic Acid 100 g/l Tin (as a methanesulfonic acid salt) 15 g/l Surfactant 0.5 ml/l pH adjusted with KOH to: 4.05

[0052] The above solution will deposit semi-bright tin at current densities of up to 20 ASF.

Example 2

[0053] A semi-bright tin-lead deposit is obtained by adding 1.5 g/l of lead methane sulfonate to the solution of claim 1 and plating at the same conditions. Ascorbic Acid 100 g/l Tin (as a methane sulfonic acid salt) 15 g/l Lead (as a methanesulfonic acid salt) 1.5 g/l Potassium methanesulfonic acid 40 g/l Surfactant 0.5 ml/l pH adjusted with KOH to: 4.05

[0054] This solution will also deposit semi-bright 90% tin at current densities of up to 20 ASF.

Comparative Example

[0055] The formulation of Example 1 was used to plate tin on 250 pieces of 8 mm diameter flat washers in a 2.5″ by 4″barrell, 140 ml of 2.5 mm diameter conductive balls were used as the media. The load was plated at 5 A, 6.5V for 15 minutes. At the end of the plating cycle, none of the flat washers were fused together.

[0056] The same plating cycle was conducted using an electrolyte of the following formulation: Citric Acid 40 g/l Tin (as a methanesulfonic acid salt) 10 g/l Lead (as a methanesulfonic acid salt) 1.5 g/l Potassium methanesulfonic acid 40 g/l Surfactant 2.5 ml/l pH adjusted with KOH to: 4.2

[0057] The load was plated at 5 A and 9V for 15 minutes at the end of the plating cycle, and only 12 pieces were not coupled together. The remaining pieces were agglomerated in groups of up to 10 pieces and were difficult to separate. This example clearly demonstrates the superiority of the solutions of the present invention. 

What is claimed is:
 1. A solution for use in connection with the deposition of one or more metals on electroplatable substrates, which comprises: water; a metal ion in an amount sufficient to provide a metal deposit on a platable substrate; a complexing agent of an organic compound having between 4 and 18 carbon atoms which compound includes at least two hydroxyl groups and a five or six membered ring that contains at least one oxygen atom, with the compound being present in an amount sufficient to complex the metal to render it soluble in the solution and to inhibit oxidation of the metal; and if necessary, a suitable pH adjusting agent to maintain the pH of the solution in the range of between 2 and
 10. 2. The solution of claim 1 wherein the complexing agent has the structure:

wherein each R is the same or different and is hydrogen or a lower alkyl group of 1 to 3 carbon atoms, T is R, OR, or O═P(OR)₂—, Z is O═ or RO—, n is 2-4 and Z can be the same or different in each occurrence in the compound, and m is 1-3, or the complexing agent is a soluble salt of such structure.
 3. The solution of claim 2 wherein the complexing agent is ascorbic acid, isoascorbic acid, dehydroascorbic acid, glucoascorbic acid, galacturonic acid, glucoronic acid, or a salt thereof, or is derived from a ketogluconate or heptagluconate and is present in an amount of about 25 to 200 g/l.
 4. The solution of claim 1 wherein the metal is tin and is added to the solution as a stannous alkyl sulfonate salt, a stannous sulfate salt, a stannous chloride salt, a stannous ascorbate salt, or stannous oxide and is present in an amount of between about 5 and 100 g/l.
 5. The solution of claim 4 further comprising a divalent lead salt in an amount sufficient to deposit a tin-lead alloy from the solution.
 6. The solution of claim 1 which further comprises a conductivity salt in an amount sufficient to increase the conductivity of the solution.
 7. The solution of claim 6, wherein the conductivity salt is an alkali or alkaline metal sulfate, sulfonate, or acetate compound.
 8. The solution of claim 1 further comprising a surfactant in an amount sufficient to enhance deposit quality and grain structure.
 9. The solution of claim 8 wherein the surfactant is an alkylene oxide condensation compound and is present in an amount of about 0.01 to 20 g/l.
 10. The solution of claim 1 further comprising an agent to promote anode dissolution.
 11. The solution of claim 10 wherein the agent to promote anode dissolution is as potassium methane sulfonate, ammonium chloride or a metal sulfide salt.
 12. The solution of claim 1, wherein the substrates are composite articles having electroplatable and non-electroplatable portions, the pH adjusting agent is an acid or a base and the pH is adjusted to the range of about 3.5 to 5.5 to enable electroplating of the electroplatable portions of the articles without deleteriously affecting the non-electroplatable portions.
 13. A solution for use in connection with the deposition of tin or tin-lead alloys on electroplatable substrates, which comprises: water; a stannous ascorbate compound; a complexing agent of an organic compound having between 4 and 18 carbon atoms which compound includes at least two hydroxyl groups and a five or six membered ring that contains at least one oxygen atom, with the compound being present in an amount sufficient to complex tin ions to render them soluble in the solution and to inhibit oxidation of the tin ions; when desired, a divalent lead compound in an amount sufficient to deposit a tin-lead alloy from the solution; and if necessary, a suitable pH adjusting agent to maintain the pH of the solution in the range of between about 2 and
 10. 14. The solution of claim 13, wherein the substrates are composite articles having electroplatable and non-electroplatable portions, the pH adjusting agent is an acid or a base and the pH is adjusted to the range of about 3.5 to 5.5 to enable electroplating of the electroplatable portions of the articles without deleteriously affecting the non-electroplatable portions.
 15. The solution of claim 13, further comprises at least one of a conductivity salt in an amount sufficient to increase the conductivity of the solution or a surfactant in an amount sufficient to enhance deposit quality and grain structure.
 16. The solution of claim 13, wherein the stannous ion is present in an amount of between about 5 and 100 g/l and the complexing agent is present in an amount of about 25 to 200 g/l and has the structure:

wherein each R is the same or different and is hydrogen or a lower alkyl group of 1 to 3 carbon atoms, T is R, OR, or O═P(OR)₂—, Z is O═ or RO—, n is 2-4 and Z can be the same or different in each occurrence in the structure, and m is 1-3, or the complexing agent is a soluble salt of such structure.
 17. The solution of claim 16 wherein the complexing agent is ascorbic acid, isoascorbic acid, dehydoascorbic acid, glucoascorbic acid, galacturonic acid, glucoronic acid, glucose-6-phosphate, or a salt thereof, or is derived from a ketogluconate or heptagluconate and is present in an amount of about 25 to 200 g/l.
 18. The solution of claim 17, wherein the conductivity salt is an alkali or alkaline metal sulfate, sulfonate, or acetate compound and the surfactant is an alkylene oxide condensation compound and is present in an amount of about 0.01 to 20 g/l.
 19. A method for electroplating a metal deposit on composite articles that include electroplatable and non-electroplatable portions which comprises contacting a plurality of such articles with the solution of claim 1 and passing a current though the solution to provide metal electrodeposits on the electroplatable portions of the articles without deleteriously affecting the non-electroplatable portions of the articles.
 20. A method for electroplating a tin or tin-lead deposit on a composite article that includes electroplatable and non-electroplatable portions which comprises contacting a plurality of such articles with the solution of claim 13 and passing a current though the solution to provide tin or tin-lead electrodeposits on the electroplatable portions of the articles without deleteriously affecting the non-electroplatable portions of the articles. 